The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous exhaust emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOX) as well as condensed phase materials (liquids and solids) that constitute particulate matter (PM). Catalyst compositions typically disposed on catalyst supports or substrates are provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
For example, reduction of NOX emissions from an exhaust feedstream containing excess oxygen is a challenge for vehicle manufacturers. By way of example, it is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOX conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOX levels. For practical application, the conversion efficiency must be obtained at a low temperature operating range (e.g., 200-350° C.) occurring during the aforementioned FTP cycle and at a higher temperature operating range (e.g., 450-550° C.) occurring during a high speed test cycle (e.g., US06 Federal Test Procedure).
The PM includes soot and other carbonaceous particulates that may be oxidized to produce gaseous CO or CO2, as well as other non-combustible particulates (i.e., ash) that are not capable of being oxidized or otherwise treated to convert them to gaseous constituents for removal from the system. The composition and morphology of PM resulting from combustion in reciprocating piston internal combustion engines is a function of the fuel, engine type, engine design, engine operation and control methodology, environmental operating conditions and other factors. For example, engine lubricating oil that passes into the combustion chamber and is partially burned produces the majority of ash. As a further example, combustion in gasoline engines may produce submicron organic matter (OM), as well as sulfates and elemental silicon, iron, or zinc or sulfur. The elemental silicon, iron and zinc are non-combustible particulates and comprise ash. As another example, combustion in diesel engines may also produce OM, sulfates and elemental silicon, iron, zinc or sulfur, as well as soot and ammonium, and may produce about 100 to about 1000 times more PM than combustion in gasoline engines. The soot may include BC (black carbon, or condensed carbon particles) and OM. The BC may be coated with a single layer or multiple layers of OM, including polycyclic aromatic hydrocarbon (PAH), as well as a layer or layers of organic or inorganic compounds. Combustion of diesel fuel may also produce nitro-PAH compounds, i.e., PAH having nitrogen functional groups. Soot from reciprocating piston internal combustion engines may have a particle size less than about 0.02 μm in diameter.
Various particulate filters (PF) for combustible particles have been employed, such as Diesel Particulate Filter devices (DPF). There are several known filter structures used in DPF's that have displayed effectiveness in removing the particulate matter from the exhaust gas such as ceramic honeycomb wall-flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall-flow filters have experienced significant acceptance in automotive applications.
The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of combustible exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration of a DPF in vehicle applications is typically automated and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF to levels that are often above 600° C. in order to burn the accumulated particulates.
One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the DPF. The HC may be delivered by injecting fuel directly into the exhaust gas system or may be achieved by late injection of fuel into the engine combustion chamber resulting in partially vaporized HC exiting the engine in the exhaust gas. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the DPF and burns the particulate accumulation. A disadvantage to this method of regeneration is that the delivery of unburned HC to the engine exhaust system reduces the efficiency of the engine/vehicle since the fuel is not being used to do useful work. Additionally, depending upon the delivery location of the HC, heat loss to the engine and the exhaust system, upstream of the DPF can be significant; further reducing the system efficiency. Also, in instances where fuel is delivered by over-fueling the engine, some fuel may bypass the pistons resulting in undesirable fuel dilution of the engine oil.
The regeneration of the DPF also is known to negatively effect SCR devices that are exposed to the regeneration temperatures by diminishing the catalytic effectiveness of SCR catalysts that are exposed to these temperatures. Thus, over time, as the number of thermal regeneration cycles increases, the conversion efficiency of the SCR diminishes. This reduction in SCR conversion efficiency over time makes it more difficult to achieve the high conversion efficiencies described above.
Thus, while various methods and apparatuses have been developed to pyrolize the combustible particulates, such as carbonaceous particulates, the regeneration process may have a negative effect on the exhaust treatment system performance. In addition, the treatment of non-combustible particulates, referred to herein generally as ash, has not been effectively addressed.
Accordingly, it is desirable to provide an effective apparatus and method for treatment of ash in exhaust gas treatment systems of reciprocating piston internal combustion engines while also maintaining the required system conversion efficiency levels for the various regulated exhaust constituents.